1. Technical Field
The present invention relates to a developing unit used in an image forming apparatus, such as a printer, a facsimile machine, a copier, etc., a process cartridge having the developing unit, and an image forming apparatus having either the developing unit alone or a process cartridge including the developing unit.
2. Related Art
An image forming apparatus with a developing unit that utilizes two-component developer including toner and magnetic carrier is widely known. To keep the density of toner in the two-component developer within a prescribed range, fresh toner is supplied from a toner container to developer stored in the developing unit as the toner is consumed during the developing process.
In such a system, since carrier included in the developer is used repeatedly although hardly consumed, either a coating (i.e., a surface layer) of the carrier is worn away or toner resins or additives adhere to the coating as image formation is repeated. As the carrier deteriorates, an amount of electric charge carried by the toner also decreases, background dirt and/or splashing of toner or the like occur. As a result, the old carrier is replaced with new carrier on a regular basis. Such maintenance increases unit price of image formation, thereby pushing up the total cost thereof.
One conventional developing unit discharges an excess amount of developer while supplying a prescribed amount of premix developer prepared by blending fresh toner and carrier to developer stored in a developing unit to restore the toner to a prescribed toner density. In such a system, old carrier is discharged little by little from the developing unit as the excess amount of developer is discharged and the new carrier included in the premix developer is supplied to the developer stored in the developing unit. Hence, since discharging and supplying of the developer gradually replaces the old carrier included in the developer with new carrier, carrier replacement can be slowed or omitted altogether.